A conventional OFDM timing and frequency synchronization method is disclosed in U.S. Pat. No. 5,732,113, issued to Timothy M. Schmidl and Donald C. Cox, entitled “Timing and frequency synchronization of OFDM signals”. FIG. 1A is a block diagram of the structure of a conventional timing and frequency synchronization apparatus disclosed in the above-described patent, and FIG. 1B is a view for illustrating the operation of the apparatus of FIG. 1A.
Referring to FIGS. 1A and 1B, in a conventional timing and frequency synchronization apparatus, a synchronizing symbol having a length of a half symbol is made up of two symbols SYN_A, a symbol SYN_B and a symbol SYN_C. A maximum point is detected by autocorrelating between the synchronizing symbol formed as described above and a delayed symbol. A symbol timing is obtained from the detected maximum point, and decimal multiple frequency offset compensation is performed. Then, an inverse Fourier transformer inverse-Fourier-transforms a received time-domain signal and the compensated received signal into a frequency domain signal. Also, integral-multiple frequency offset compensation is performed using a differential signal obtained by differentially encoding the synchronizing symbols A and B.
However, the above-described conventional method has a problem in that the probability of an error occurring during obtaining symbol timing is high since a variation in the maximum point of an autocorrelation value is great due to the influence of noise in a channel. Also, fine frequency synchronization and coarse frequency synchronization depend on symbol timing synchronization, so that they are sensitive to the influence of symbol timing errors. Furthermore, in the above-described conventional method, a received signal stored in a memory, and a current received signal are both inversely Fourier transformed, which causes complexity.
Meanwhile, a broad-band wireless LAN uses a 20 MHz frequency band and 64 sub-carriers, and a maximum frequency offset is set to be 200 kHz. Thus, a broad-band wireless LAN does not consider a frequency offset which corresponds to an integral multiple of a sub-carrier frequency. However, the conventional frequency and symbol synchronization method of OFDM signals considers an integral-multiple frequency offset, so that it is not efficient.